The present invention relates primarily to nonvolatile, magnetic, solid-state memories which employ the principles of magnetoresistance. An overview of solid-state memory technologies--including electrical, magnetic, optical molecular, chemical, and biological--is given by Ashton in the Proceedings of the "Conference on Solid-State Memory Technologies", Pasadena, Calif., 23-25 May 1994, and by Ashton et al. in "Solid-State Memory Study", Technical Report RE-0013, National Media Lab, February 1994, both of which are incorporated herein by reference. The present invention relates furthermore to an array of memory cells, each cell including storage, read, and write elements. More specifically, the magnetic storage elements of the present invention employ a closed-flux structure and operate without the magnetization being rotated away from the direction of the flux closure. This results in several advantages discussed herein.
Magnetic mass storage devices with moving disk or tape are commonplace in the computer industry. A description of such magnetic storage devices, as well as some of the operational problems and limitations are described in U.S. Pat. No. 5,237,529, the entire specification and claims of which are incorporated herein by reference. Although such magnetic mass storage devices provide substantially permanent storage of information, they typically have relatively long access times, are sensitive to shock and vibration, and have tribological constraints, i.e., wear and friction.
Thin-ferromagnetic-film random-access memories are nonvolatile and have fast random access. Ferromagnetic film memories in the 1960 era used inductive readout. Inductive readout has been abandoned because the signal becomes too small when the element is miniaturized. Modern magnetic film memories use anisotropic magnetoresistive readout. Anisotropic magnetoresistance (AMR) is proportional to the square of the sine of the angle between current and the magnetization vector. Readout requires the magnetization vector to be rotated so that a component is perpendicular to the rest position. Closed-flux operation would require closing the flux in two orthogonal directions around insulated word and digit conductors, a problem that has never been solved. Consequently, such memories are operated with flux closure in one direction only, and rotate the magnetization into a nonclosed configuration. This leads to many difficulties such as information loss due to magnetization creep, high current-drive requirements, and low signal. This is discussed in U.S. Pat. No. 5,251,170 to Daughton and Pohm, the entire specification of which is incorporated herein by reference. The smaller the film element, the greater the demagnetizing field, and the greater the difficulties.
Giant magnetoresistance (GMR) is very different from AMR. GMR can be an order of magnitude larger than AMR, and it appears only in inhomogeneous materials, especially layered structures. For GMR, the change in resistance is proportional to the cosine of the angle between the magnetization in one region and the magnetization in the other. This is different from the AMR sine of twice the angle between the current and the magnetization. Consequently, in GMR the maximum signal difference occurs when the magnetization of one layer is changed by 180 degrees. With AMR this produces no change. Thus, with a GMR memory element, it is possible to operate with the magnetization restricted to one axis, and flux closure only along that axis is required.
GMR (sometimes called the spin valve effect), is discussed in a number of patents, mostly for the application of read heads for disks or for magnetic field sensors. Dieny et al. (U.S. Pat. Nos. 5,159,513 and 5,206,590) discuss a GMR sensor consisting of two magnetic layers separated by a thin film of Cu, Au, or Ag. One of the magnetic layers is high-coercivity Co. The magnetizations in the two magnetic layers are perpendicular to each other for maximum sensitivity. Sakakima et al. (U.S. Pat. No. 5,243,316) discusses the materials for an improved GMR element for magnetic sensing and for thin-film heads. In this patent, the high-coercivity material is Co-rich FeCo, and the low-coercivity material is Ni-rich NiFeCo. Cain et al. (U.S. Pat. No. 5,301,079) describes a read head in which two low-anisotropy magnetic films with easy axes aligned are separated by a current-bearing nonmagnetic conducting film. Current in that nonmagnetic film rotates the magnetization in the two layers in opposite directions to achieve maximum sensitivity to fields from domains in a disk. Saito et al. (U.S. Pat. No. 5,304,975) discusses magnetoresistive sensors consisting of many periods of alternately stacked magnetic and nonmagnetic layers. Included is a layer to apply a magnetic bias to assist reversal.
A. V. Pohm and C. S. Comstock published a paper entitled "Memory Implications of the Spin-Valve Effect in Soft Multilayers" (J. Appl. Phys 69, 5760, 1991). Their discussion is of a GMR memory element having a sense-digit line perpendicular to a word line. The element has a closed structure in one axis only, and the magnetization is rotated out of the axis of flux closure by current in the word line. This results in higher current requirements and a risk of information loss through magnetization creep.
Kung et al. (U.S. Pat. No. 5,343,422) teach a GMR memory element. This comprises a substrate and a rectangular multilayered structure deposited thereon which includes two layers of ferromagnetic material separated by a layer of nonmagnetic metallic conducting material. The magnetization easy axes of the two magnetic films are parallel. The magnetization of one of the ferromagnetic layers is fixed, and the magnetization of the other is free to change direction between the "one" state and the "zero" state. This is done with two striplines, a word line and a sense-digit line. These two striplines are perpendicular to each other. This invention has no provision for closed-flux structure; instead it relies on the width of the element being so small that it cannot support a domain wall, so each individual film cannot become demagnetized. In addition, each individual memory element has two semiconductor gates, so the elements don't ever have to suffer half select pulses. One disadvantage of having two gates per element is the cost in real estate per element. The total capacity per chip is diminished.
Semiconductor random access memory (RAM) is also well known to those of skill in the art. RAM generally comprises a set of memory cells integrated on a chip with a number of peripheral circuits. RAMs are described in, for example, Porat et al., Introduction to Digital Techniques, John Wiley, 1979, the entirety of which is incorporated herein by reference. In general, RAM circuits perform several functions, including addressing (selection of specific locations for access), providing power, fanout (transmission of a signal to a multiplicity of loads), and conditioning required to generate a useable output signal. In RAM memories, the addressing scheme permits random access to the desired cell, with access time being independent of the cell location. Selected portions are then extracted for use. RAMs are generally fast enough to be compatible with a CPU, but they are generally too expensive to be used for mass storage. Further, both static RAMs (SRAMs) and dynamic RAMs (DRAMs) are volatile in the sense that their contents are lost when the power to the memory is lost. DRAMs also require periodic refreshing. It is not practical, therefore, to use either DRAMs or SRAMs for long-term storage.
Electronically programmable read only memory (EPROM) and read only memory (ROM) are nonvolatile alternatives to RAM. However, while such memories do not require a refresh cycle, they have the obvious disadvantage of being programmable only once. Other nonvolatile semiconductor memories that can be written repeatedly, such as electrically alterable read only memory (EAROM) or electrically erasable read only memory (EEROM), or FLASH (an application-optimized EPROM), do not provide nearly the reliability of magnetic memories for long-term storage.
From the foregoing, it is evident that an improved memory which provides the random access, speed, and ruggedness of RAMs, but is nonvolatile and does not require either a standby or periodic-refresh power source is desirable for permanent storage applications.